This invention relates to a linear positional displacement detector and, more particularly, to a linear positional displacement detector for detecting a linear displacement of a permanent magnet as a change in direction of magnetic flux on the sensing surface of a magnetic sensor unit.
FIG. 21 is a sectional side view of a conventional linear positional displacement detector disclosed in Japanese Utility Model Laid-Open No. 2-36089 and FIG. 22 is a sectional plan view of the conventional displacement detector illustrated in FIG. 21.
In these Figures, the linear displacement detector comprises a case 1 having end plates 2 having shaft bores 2a into which an elongated shaft rod 3 is slidably inserted and a cover plate 4 fitted in an upper opening of the case 1. In operation, the shaft rod 3 is connected to an apparatus (not shown) whose physical movement is to be detected. The case has mounted on an inner surface of the cover plate 4 a magnetic sensor 7 having a pair of magnetoresistance elements 6a and 6b (FIG. 23) through an electrically insulating plate 5. The case 1 also supports a pair of parallel guide rods 8 at leg portions 4a of the cover plate 4.
Slidably mounted on the guide rods 8 is a slider 9 having a permanent magnet 10 having an elongated and slanted magnetic pole face. The slider 9 is connected to a boss 11 secured to the shaft rod 3 through a substantially U-shaped coupling 12, so that the slider 9 having the permanent magnet 10 is linearly moved along the guide rods 8 when the shaft rod 3 is moved in the axial direction. A compression spring 13 is disposed between the end plate 2 of the case 1 and the boss 11 in order to elastically hold the slider 9 in its home position when no external force is applied to the shaft rod 3.
FIG. 23 illustrates the positional relationship between the magnetoresistance elements 6a and 6b of the magnetic sensor unit 7 and the permanent magnet 10 on the slider 9. The permanent magnet 10 has a magnetic pole face exposed on the upper surface of the slider 9 and the magnetoresistance elements 6a and 6b have their magnetic sensing surfaces exposed in the bottom surface of the sensor unit 7, so that the pole face of the permanent magnet 10 and the magnetic sensing surfaces of the magnetoresistance elements 6a and 6b are positioned in a facing parallel relationship to each other and the permanent magnet 10 is movable relative to the magnetoresistance elements 6a and 6b while maintaining the above-mentioned parallel facing relationship. Also, the permanent magnet 10 is slanted relative to the direction of movement and the magnetoresistance elements 6a and 6b are similarly slanted.
In FIG. 24a, when the permanent magnet 10 is moved in the direction of an arrow A from the illustrated position in which the entire magnetic sensing surface of the first magnetoresistance element 6a faces or overlaps the pole face of the permanent magnet 10, the area or portion of the magnetic sensing surface of the first magnetoresistance element 6a that is in the facing relationship with the permanent magnet 10 gradually decreases and instead the area or portion of the magnetic sensing surface of the second magnetoresistance element 6b that is in facing relationship with the permanent magnet 10 gradually increases.
In FIG. 24b, when the permanent magnet 10 is moved in the direction of an arrow B from the illustrated position in which the entire magnetic sensing surface of the second magnetoresistance element 6b faces or overlaps the pole face of the permanent magnet 10, the area or portion of the magnetic sensing surface of the second magnetoresistance element 6b that is in the facing relationship with the permanent magnet 10 gradually decreases and instead the area or portion of the magnetic sensing surface of the first magnetoresistance element 6a that is in facing relationship with the permanent magnet 10 gradually increases.
Thus, as the permanent magnet 10 makes a linear displacement between the positions shown in FIGS. 24a and 24b, the facing areas of the magnetic sensing surfaces of the first and the second magnetoresistance elements 6a and 6b that are in the facing relationship with the permanent magnet 10 change. This change in the facing area causes a change in the magnetic flux perpendicularly passing through the sensing surface which causes the resistance value of the first and the second magnetoresistance elements 6a and 6b to change. Therefore, by detecting the resistance value of the magnetoresistance elements 6a and 6b, the displacement of the permanent magnet 10 can be detected. In the arrangement illustrated in FIGS. 24a and 24b in which the permanent magnet 10 and the magnetoresistance elements 6a and 6b are slanted by an angle .theta. with respect to the direction of movement of the magnet 10 and the magnetoresistance elements 6a and 6b have width dimension of L, linear output characteristics can be obtained in so far as the permanent magnet 10 moves within a range or a distance L/sin.theta. which may be referred to as a linear detection range.
In the conventional linear positional displacement detector as above-described, the linear detection range which is the distance of movement of the permanent magnet 10 over which the linear output characteristics is obtained is relatively large. However, this detection range is sometimes unsatisfactory according to the application and it is desired that a linear positional displacement detector which has a larger linear detection range be developed.